Methods for producing methacrylic acid include a method in which isobutylene, tertiary butyl alcohol, methacrolein, or isobutyraldehyde is subjected to catalytic gas phase oxidation with molecular oxygen in a one- or two-stage reaction. In addition to methacrylic acid (boiling point 161° C./760 mmHg, melting point 15° C.), the target substance, for example, carboxylic acids such as formic acid, acetic acid, propionic acid, maleic acid, citraconic acid, benzoic acid, toluic acid, terephthalic acid, and acrylic acid, and aldehydes such as formaldehyde, acetaldehyde, propionaldehyde, methacrolein, benzaldehyde, tolualdehyde, and furfural are contained as by-products in a product obtained by this method.
Most of these impurities are separated by purification means such as an extraction operation, a distillation operation, or a combination thereof, and methacrylic acid having a certain level of purity can be obtained.
Methods for purifying methacrylic acid include a crystallization method in addition to an extraction method and a distillation method.
Patent Literature 1 describes a method in which methanol, ethanol, propanol, butanol, or the like as a second component is added to crude methacrylic acid, methacrylic acid is crystallized from this solution, and the precipitated crystals and the mother liquor are separated to obtain purified methacrylic acid.
Patent Literature 2 describes a method of performing crystallization with a solution obtained by adding methanol as a second component to crude methacrylic acid, using a continuous crystallization apparatus in which a first crystallization vessel and a second crystallization vessel are connected, and in the method, methacrylic acid is crystallized in a state in which some of the suspension slurry in the second crystallization vessel is circulated to the first crystallization vessel, and crystals precipitated in a purification column and mother liquor are separated to obtain purified methacrylic acid.
Patent Literature 3 describes a method in which crystallization is performed using a scraper unit having a structure that effectively suppresses the growth of a crystal layer on a cooling surface, thereby stably obtaining purified methacrylic acid for a long period.